A team of scientists from the University of the Witwatersrand have discovered the first evidence of a comet's impact on Earth – a small black pebble filled with diamonds.

"Comets always visit our skies – they're these dirty snowballs of ice mixed with dust – but never before in history has material from a comet ever been found on Earth," said Professor David Block of Wits University.

"Comets contain the very secrets to unlocking the formation of our solar system and this discovery gives us an unprecedented opportunity to study comet material first hand."

The pebble was found in an area of fused glass, the Libyan Desert Glass area spread over around 6,000 square kilometers, and attracted the eye of a geologist because of its unusual features and the lack of any corresponding rock formations. A laboratory analysis showed the rock was very unusual indeed.

While the bulk of the pebble's material is carboniferous, with a few silicates mixed in, the scientists found that interspersed with the material of millions of tiny diamonds. These are usually found on Earth in rocks that have been subjected to very high pressures, but the scientists posit that the same materials could be generated in an exploding comet.

According to the paper published in Earth and Planetary Science Letters, around 28 million years ago a comet entered the Earth's atmosphere over Egypt and then exploded. The pressure of the explosion formed the diamonds in the pebble and the resulting heatwave created the Libyan Desert Glass.

The central scarab in Tutankhamen necklace is the result of the comet's impact

The glass itself has been valued by humans since the Pleistocene era and is featured in jewelry owned by the late boy-king of Egypt Tutankhamen. Geologists had thought that the glass was formed by a meteorite impact but the new evidence suggests a comet is to blame, and the evidence could also save space agencies a few pennies.

"NASA and ESA (European Space Agency) spend billions of dollars collecting a few micrograms of comet material and bringing it back to Earth, and now we've got a radical new approach of studying this material, without spending billions of dollars collecting it," said the paper's author Jan Kramers.

Many comets are thought to have slammed into Earth's atmosphere – the 1908 Tunguska Event is widely assumed to be a comet that also exploded in an air-burst. But the pebble, called "Hypatia" after the first recorded female mathematician Hypatia of Alexandria, is the first direct proof of such extraterrestrial material raining down on Earth. ®